In the process of electrostatographic reproduction, a light image of an original to be copied or printed is typically recorded in the form of a latent electrostatic image upon a photosensitive member with a subsequent rendering of the latent image visible by the application of electroscopic marking particles commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support medium such as a sheet of plain paper. To render this toner image permanent, the image must be “fixed” or “fused” to the paper, generally by the application of heat and pressure.
With the advent of high speed monochrome and color marking machines, including xerography reproduction machines wherein copiers or printers can produce at a rate in excess of three thousand copies per hour, the need for improved developer performance is evident and useful.
A common goal in the design and development of electrostatographic marking devices is the ability to maintain optimum image quality from page to page and job to job regardless of the characteristics of the images being formed on each page. As should be appreciated, to maintain optimum image quality, it is important that the printing device sustain good quality developer, good development as well as good transfer efficiency. Good development or good developability refers to the ability of the device to transfer the appropriate amount of high quality toner to the latent image when forming the toner powder image.
It is known that maintaining the state of the material in the developer housing within an optimum purity range improves developability and transfer efficiency. To accomplish this, many marking systems use a variety of processes to maintain the state of the developer materials within the optimum range by monitoring and controlling one or more characteristics of the materials including, for example, temperature, humidity, charge, toner concentration (ratio of toner to carrier), toner purity and toner charge distribution.
However, even if the developer materials are maintained in an optimal state, it has been observed that under certain conditions such as extended running of prints with lower toner area coverage in one or more of the color separations, the developability and/or transfer efficiency can degrade and become contaminated due to changes in the material's state in the developer housing. Foreign particles in the toner and degradation in developability and/or transfer efficiency produces weak, mottled and/or streaky images.
The analytical test that measures the amount of foreign particles and coarse toner particles in a sample of toner has been standard for many years. This test is important because the analysis links the quality of the toner to the type of print defect. This was the necessary and sufficient test that provided feedback to toner manufacturing operations to verify that quality toner is being produced. More recently, however, there has been a print defect identified that is caused by agglomerated toner or additive powder. A gently screening process was developed that is able to isolate these soft agglomerated toner particles. Unfortunately, quality problems with toners cannot be captured by this test because these new type of soft agglomerated toner particles are too friable. The particles break up with the required mechanical screening and brushing such that there are little if any retains remaining. If low levels of vacuum are used to pull material away from the screen, then even fewer retains or contamination particles can be expected resulting in a misleading analytical test result. The consensus of studying the problem is that the current analytical test is not able to segregate the soft agglomerates that are causing the print defect problem. Despite adjustments with different combinations of sample sizes, vibration, screen sizes, vacuum and brushing, these particles cannot be reliably removed and captured. So, without a reliable analytical test method, it is a great risk to begin toner manufacturing production.
In electrostatic development processes, a developer material is used comprising relatively large magnetic carrier beads that have fine toner particles electrostatically attracted to and coated thereon. Various known means are used to convey these toner particles to the latent electrostatic image on the photoconductive surface. The composition of the carrier particles is so chosen as to electrostatically attract and hold the toner particles for transfer to the latent image, preferably without contaminants. As the developer is directly or indirectly contacted into this photoconductor surface, the toner particles are electrostatically deposited and secured to the charged portion of this latent image and not deposited on the uncharged or background portion of the image. The carrier and excess toner are then recycled for later use but eventually, after extended use, become contaminated and ready to be removed from the system to be replenished with new toner and carrier. A system to extend developer life and purify the toner would be extremely economically attractive.
In magnetic brush development, bad particles, impurities, contaminants or agglomerates in the toner will separate out during development and cause dots or spots on the paper or receiving member thereby ruining the final copy. These agglomerates or impurities do not attach to the carrier because they are either or both too large in size or they do not possess a strong enough opposite tribo charge to the carrier charge. Using this knowledge, a magnetic purification process of the present invention can work in a similar fashion simulating this development process.